1. Field of the Invention
This invention is concerned with a method for recovery of petroleum from viscous petroleum-containing formations including tar sand deposits, and more particularly is concerned with an improved combined miscible flooding noncondensible gas injection technique especially useful in viscous petroleum-containing fromations including tar sand deposits.
2. Description of the Prior Art
There are many subterranean petroleum-containing formations in various parts of the world from which petroleum cannot be recovered by conventional means because the petroleum is too viscous to flow or be pumped. The most extreme example of viscous petroleum-containing formations are the so-called tar sand or bituminous sand deposits. The largest and most famous such deposit is the Athabasca Tar Sand Deposit in the northeastern part of the Province of Alberta, Canada, which is estimated to contain over 700 billion barrels of petroleum. Other extensive deposits are known to exist in the western United States and Venezuela, and smaller deposits exist in Europe and Asia.
Tar sands are defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum constituent of tar sand deposits are highly bituminous in character. The sand content of tar sand deposits are generally fine quartz sand coated with a layer of water, with the bituminous petroleum material occupying most of the void space around the water wetted sand grains. The balance of the void space is filled with connate water, and some deposits contain small volumes of gas such as air or methane. The sand grains are packed to avoid volume of about 35 percent, which corresponds to 83 percent by weight sand. The balance of the material is bitumen and water, and the sum of bitumen and water is fairly constantly 17 percent by weight, with the bitumen portion thereof varying from about 2 percent to about 16 percent. One of the characteristics of tar sand deposits which differs considerably from conventional petroleum-containing formations is the absence of a consolidated mineral matrix within the formation. The sand grains are in contact although uncemented and the bitumen occupies most of the void space. The API gravity of the bitumen ranges from about 6 to about 8, and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027.
The methods for recovering bituminous petroleum from tar sand deposits include strip mining and in situ processes. Most of the recovery to date has been by means of strip mining, although this is economically feasible only when the ratio of overburden thickness to tar sand deposit thickness is around 1 or less. Vast quantities of petroleum are known to exist in the form of tar sand deposits which are not within a range which is economically suitable for strip mining, and so some form of in situ processes wherein the bitumen or bituminous petroleum is separated from the sand by some means and recovered therefrom by a well or other production means drilled into the tar sand deposit is needed.
In situ processes may be categorized as thermal techniques, including fire flooding and steam flooding, and emulsification drive processes. Emulsification processes frequently also employ steam, plus a basic material such as sodium hydroxide which induces formation of an oil-in-water emulsion having a viscosity substantially lower than the viscosity of the formation petroleum. Thermal processes are suitable for use only in formations having sufficient overburden thickness to permit injection of high pressure fluids. Many tar sand deposits exist in which the overburden thickness is too thin for thermal flooding and too thick for strip mining.
One process for recovering bitumen from tar sand deposits which does not require the presence of sufficient overburden thickness to contain high pressures is solvent flooding. Solvent flooding involves injection of a solvent into the tar sand deposit, which solvent dilutes and reduces the viscosity of the bituminous petroleum to render it mobile and recoverable by means of a well as is normally employed in conventional oil recovery operations. Although laboratory experiments can amply demonstrate that many solvents including aromatic hydrocarbons such as benzene and toluene, as well as carbon tetrachloride or carbon disulfide, can readily dissolve bituminous petroleum, there are yet problems in some formations which must be solved if commercial exploitation of tar sand deposits by solvent flooding is to be feasible.
The most desirable type of oil recovery operation, including a solvent injection oil recovery process, is the multi-well throughput operation in which a fluid is injected continuously into one or more injection wells and a mixture of the fluid and the petroleum is recovered continously from one or more spaced apart production wells. When solvent is injected into an injection well and begins to move away from the point of entering the petroleum formation adjacent to the injection well, bitumen is dissolved in the solvent; the fluid moving through the formation is the solvent-bitumen solution. The amount of bitumen dissolved in the solvent increases with contact time, and so the solvent-bitumen solution moving through the formation in a general direction of the production well dissolves increasing amounts of bitumen. Since bitumen is an extremely viscous material, it frequently occurs that a solution containing the maximum concentration of bitumen which will dissolve in a given solvent becomes too viscous to flow in the formation. This phenomenon gives rise to the formation of a bitumen bank, or a bank of bitumen and solvent moving like a wave through the formation, but with its viscosity increasing continually as it moves away from the injection point. Eventually, the bitumen-solvent blend viscosity increases to a point where it will no longer move under the available pressure gradient, after which further injection of the solvent into the formation becomes impossible. Although it would be possible to reduce the viscosity of the bitumen bank by contacting it with additional solvent, the sovlent cannot reach bitumen where it is needed most. The receptivity of the formation to solvent decreases dramatically because of the plugging phenomenon without realization of the benefits desired by the injection of solvent into the formation.
It can be seen from the foregoing that there is a substantial need for a method of preventing the formation of a highly viscous, immobile bitumen or bitumen-solvent plug in a tar sand formation. Stated in another way, there is a need for maintaining flow channels open within the formation so that the injected solvent can contact and dissolve bitumen and transport it to the production well.